A set of internationally accepted units to aid in communication of measurements.

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Unification: what does it mean for the forces to become equal?

The Coulomb force constant $k_c$ has si-units of $Nm^2C^{-2}$. The gravitational force constant $G$ has si-units of $Nm^2kg^{-2}$. What does it mean that at very high energies these forces become ...
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4answers
87 views

What forms can units take?

They have stated in my physics book that all units can be made by combining SI base units. I have got a few question about this. Can we raise one unit to the power of another unit? For instance: ...
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2answers
57 views

Intitutive meaning the unit of Force $N$

I infer 1 Newton of force = 1 kg.m/ sec squared means a force if acting continuosly on a body at rest would make it gain an acceleration of 1 m/sec squared each second.So the change in velocity is 1-0 ...
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3answers
186 views

Is the speed of light in vacuum constant or does the math just happen to work out?

My apologies if my question is really idiotic, but I ask sincerely because I want to learn. Based on this question and lots of other places on the web, this topic seems to be really confusing. Let's ...
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0answers
19 views

Why is Ampere not a derived SI unit? [duplicate]

From my understanding, Ampere is amount of elementary charge per time. So, you should be able to derive it from mol, which is amount, and seconds, which is time. My question: Why is Ampere a base ...
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2answers
86 views

What is the meaning of SI unit $Ns$ (NEWTON.TIME)?

for ex: when i say newton per meter it mean that 1 newton is applied per 1 meter. but what i mean by saying N.s (s=time in sec) i.e Impulse or momentum. N.s does not mean that certain force is being ...
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3answers
150 views

Messed up units!

In the article, Environment-assisted quantum transport, $\gamma$ is a constant equal to $2\pi kT/\hbar*E_{R}/(\hbar\omega_{c})$ where $T$ is the temperature, $k$ is the Boltzmann constant. ...
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2answers
86 views

What is $c$ in the Lorentz force expression?

The usual Lorentz force expression I am familiar with is this: $$\vec F=q(\vec E+\vec v \times \vec B)$$ I have seen some other versions lately that include an extra factor $1/c$: $$\vec ...
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1answer
129 views

What is the reason we originally and still use the non-SI unit, the Jansky?

The Jansky is the unit for spectral flux density. It is defined as $$1 {\rm \ Jy} = 10^{-26} {\rm W \ m^{-2} \ Hz^{-1}}$$ in terms of Watts per square meter per Hertz. I've never quite understood ...
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1answer
37 views

Mass-energy-equivalence's effect on SI units?

Maybe I'm misunderstanding mass energy equivalence, but can we use it to get rid of the Joule (or the kilogram) and have a single unit for both? It seems weird that if they're equivalent they'd have ...
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3answers
108 views

Why is candela a base SI unit?

The candela is defined as The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency $540\cdot10^{12}$ hertz and that has a radiant ...
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2answers
414 views

Is the second defined arbitrarily? [duplicate]

According to the definition a second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of ...
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2answers
866 views

What is the fundamental units for coulomb?

Everything I saw suggests coulomb has no fundamental units. So then how is Newton/Coulomb equivalent to Volt/meter?
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3answers
1k views

Why is the mole/“amount of substance” a dimensional quantity?

According to the BIPM and Wikipedia, "amount of substance" (as measured in moles) is one of the base quantities in our system of weights and measures. Why? I get why the mole is useful as a unit. In ...
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2answers
61 views

Usage of singular or plural SI base units when written in both symbol as well as name [closed]

I have multiple doubts related to the usage of singular or plural SI base units when written in both symbol as well as name. I have framed this question under two parts, namely, Part (a) and Part ...
43
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5answers
6k views

Why is the prospective new kilogram standard a sphere?

I can understand the choice of material, silicon 28, but why is it a sphere rather than (say) a cube? Article here I would have thought that a sphere would have been the hardest shape to machine ...
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0answers
38 views

How is the Joule normalised acrpss various definitions?

Apologies if this question is a duplicate, I tried searching for this question both on Google and here, but was unable to find an answer. A Joule is defined in various ways, some of them being: ...
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2answers
53 views

Convert cubic feet per sec to feet per sec

Is there a way to convert cubic feet per sec to feet per sec. Or in general volumetric flow to velocity? I want to know the time taken by water to travel from point A to point B. I have the distance ...
20
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5answers
4k views

Why is energy not an SI base unit?

According to a textbook I have begun to read, there are seven base SI units: Length Mass Time Temperature Amount of a substance Electric current Luminous intensity What I do not understand is, why ...
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0answers
49 views

Why is an Ampere an SI unit? [duplicate]

It has always annoyed me that an Ampere is an SI unit, rather than a Coulomb. Why is this the case? Was current discovered first historically? I believe that the standards were published in the 1960s, ...
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2answers
602 views

Where does the joules come from (ideal gas constant)?

I have not done any physics in ages and have recently started studying it. The first chapter in my book deals with the ideal gas constant: $$pV=nRT$$ It is rewritten as: $$R=\frac{pV}{nT}$$ When I ...
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4answers
4k views

Why are grams usually only expressed as milligrams, grams or kilograms?

I'm a physics (and electronics and astronomy, etc.) enthusiast. As I learn and research topics, I notice that many SI units are often expressed using a variety of prefixes, such as in electronics ...
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137 views

Why was the original definition of meter abandoned?

The meter was originally intended to represent $1$ ten-millionth($10^{-7}$) of the distance from pole to equator of the Earth along a meridian of longitude. The definition was later discarded. ...
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1answer
66 views

A query regarding constant $\frac{1}{4\pi \epsilon_{0}}$ in Coulomb's Law [duplicate]

In our physics class we were discussing about Coulomb's Law and equation for the electrostatic force between two points: $$F_{e}=\frac{Q_{1}Q_{2}}{4\pi \epsilon_0 r^2}$$ From the equation a query ...
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259 views

Numerically solving 2D poisson equation by FFT, proper units

The 2D Poisson equation is: (1)$$\frac{d^2\varphi(x,y)}{dx^2}+\frac{d^2\varphi(x,y)}{dy^2}=-\frac{\varrho(x,y)}{\epsilon_0\epsilon}$$ And in $k$-space it is in form of: (2)$$(k_x^2+k_y^2) ...
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447 views

Speed of light definition

I understand the speed of light to be an EXACT number. If permeability of free space has a factor of pi in it, how can this be?
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1answer
89 views

Why is the Fermi coupling constant always expressed in units of $(\hbar c)^3$?

Everywhere I've looked so far (such as NIST) the Fermi coupling constant $G_F$ is always expressed as $$\frac{G_F}{(\hbar c)^3} = 1.166 364(5) \times 10^{-5} \textrm{ GeV}^{-2}$$ never as just ...
21
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1answer
464 views

What are the proposed realizations in the New SI for the kilogram, ampere, kelvin and mole?

The metrology world is currently in the middle of overhauling the definitions of the SI units to reflect the recent technological advances that enable us to get much more precise values for the ...
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1answer
83 views

In the expresion $4\sin (\theta) \frac{d \theta}{d t}=\frac{dy}{dt}$ Which is the correct unit of $\theta$ ¿ radian or degrees? [closed]

If I change units of an angle for radians to degrees in the next expresion $$4\sin (\theta) \frac{d \theta}{d t}=\frac{dy}{dt}$$ the value of $$\frac{dy}{dt}$$ changes. For example at a rate of ...
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2answers
353 views

Why aren't Faraday's law of induction and Maxwell-Ampere's law (without sources) symmetric?

I was wondering why Faraday's law of induction and Maxwell-Ampere's law (without sources) are not totally symmetric in the sense that Maxwell-Ampere's law has a $\epsilon_0 \mu_0$ term on the right ...
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1answer
153 views

Dimensional Analysis in Electromagnetism (SI vs Gaussian-cgs)

Looking at Konopinski's formula for conjugate momentum (in the comment after equation 3 of "What the Vector Potential Describes"): p = M v + q A /c it is plain enough that M v is momentum, but if we ...
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2answers
150 views

25 cm of vacuum correspond to what pressure?

I the following (american) test Vacuum testing consists of placing samples from the packiging operation into a jar filled with water. A lid is placed over the samples to fully immerse them in the ...
2
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1answer
79 views

What was a second in the early universe?

I have read some popsci articles and documentaries about the early universe and they often explain how various features of the universe came about and at what time. For example hydrogen atoms came ...
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4answers
541 views

Is there a quantity measured in kilogram seconds?

I'm trying to get a full grasp on the relationship between many of the units that are used in kinetic physics. I've found that it is possible to make a venn-diagram that shows the factors of many of ...
2
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0answers
46 views

Different factors of $4\pi$ and $\epsilon_0$ in Poisson equation [duplicate]

Some authors claim the Poisson equation is $$\nabla^2 \psi = -\dfrac{\rho}{\epsilon\epsilon_0}$$ (e.g. Wikipedia) whereas other ones (e.g. Andelmann) claim it is $$\nabla^2 \psi = ...
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3answers
245 views

Does the definition of the SI unit “second” require that possible perturbation of primary frequency standards should be measured?

The definition of the SI unit "second" is stated as The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground ...
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3answers
114 views

How units were defined?

I was wondering how we humans can be sure that one meter is one meter and that one second is one second. Nowadays, except for the Kilogram, all other units are well defined using highly accurate ...
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3answers
1k views

Why is the second the SI base unit for time?

Specifically, during the moves towards Le Système international d'unités in the 18th and 19th centuries, why didn't anyone attempt to move us away from the definition of there being 24 hours in a day? ...
3
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1answer
620 views

Why is charge not taken as a fundamental unit? [duplicate]

According to the definition of electric current, it appears to be a derived quantity. Charge on the other hand seems more fundamental than electric current. Then why is current taken as fundamental ...
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2answers
425 views

How do we convert between the maximum value of the adhesive force (in kilograms or in Newton) of a magnet and its magnetic field strength (in tesla)?

On the following website they indicate how powerful is a magnet by giving the theoretical maximum value of its adhesive force in kilograms or in Newton (its adhesive force at a distance of zero). ...
2
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1answer
322 views

Are there reasons for the discrepancies in absolute temp units - Kelvin vs. kelvins vs. degrees Kelvin?

Before 1968, the units for absolute temperature were described as "degrees Kelvin" or "degrees absolute." After that, the SI system got rid of the idea of "degree" for absolute temperature, so the ...
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3answers
630 views

Why didn't we replace our SI units with a better system? [closed]

Intro It seems to me that the SI units we use today are nothing but the result of a historical 'coincidence'. I recently began researching about natural (absolute) systems of units, which are ...
0
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1answer
267 views

Definition of metre

We know that 1 meter is the distance travelled by light in vacuum within a time interval of 1/299,792,458 second. My question is why we didn't take a simpler number like 1/300,000.000 or why not just ...
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2answers
99 views

Why is the Coulomb taken as the unit positve test charge, though $1 C$ charge is a high value? [closed]

Why is the Coulomb taken as the unit positve charge, though $1 C$ charge is a high value?
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2answers
422 views

Uncertainty of permittivity of vacuum [duplicate]

Question: The value of permittivity of vacuum, $\epsilon_0$, is given with absolutely no uncertainty in NIST Why is this the case? More details: The permeability of vacuum can be given by ...
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1answer
285 views

Why does milli- mean 1/1000 [closed]

I suppose this is also an English question, but I'll ask it here first. Why does the milli- prefix mean 1/1000 when it sounds so much like million? According to the internet, this dates back to the ...
0
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1answer
86 views

The value of one atomic mass unit

In my textbook, it is given: 1 atomic mass unit equals $1/12$th mass of one carbon 12-atom. Since mass of $6.02 * 10^{26}$ atoms of carbon-12 is $12\space\text{kg}$. Thus, $$1 \text{ a.m.u ...
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2answers
2k views

What is the relationship between keV and kVp?

On X-ray topics we get used to talk about "energy", but what is keV? and what is the relationship between keV and kVp?
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272 views

What is dimensional units/quantity and dimensional state

First, I am not a native English-speaking student so I am not good at physics definitions in English. I participated in the MIT e-learning course on classical physics. The 1st lesson is about 3 ...
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1answer
1k views

Is sound considered a subtopic of physics? What are the SI units of sound?

Does physics cover sound? If so, why does sound not have units in the SI system, or how would we measure sounds and frequency? I guess it's debatable, but why isn't it standard practice in physics?